Dielectric ceramic and laminated ceramic capacitor

ABSTRACT

A dielectric ceramic which improves the lifetime characteristics and dielectric breakdown voltage of a laminated ceramic capacitor includes core-shell crystalline grains which have a core-shell structure and homogeneous crystalline grains which have a homogeneous structure. In this dielectric ceramic, the core-shell crystalline grains and the homogeneous crystalline grains are present at an area ratio in the range of 91:9 to 99:1. Preferably, when the mean grain size for the core-shell crystalline grains is represented by R1 and the mean grain size for the homogeneous crystalline grains is represented by R2, the ratio of R2/R1 is 0.8 or more and 3 or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a dielectric ceramic, and a laminated ceramiccapacitor configured with the use of the dielectric ceramic, and moreparticularly, relates to an improvement for making the withstand voltageof a dielectric ceramic higher.

2. Description of the Related Art

As one of effective means for satisfying the demands of reduction insize and increase in capacitance for laminated ceramic capacitors, anattempt to make dielectric ceramic layers included in laminated ceramiccapacitors thinner may be made.

However, making the dielectric ceramic layers thinner also makes iteasier to cause dielectric breakdown in laminated ceramic capacitorswhen a large direct current or the like is applied. Given thesecircumstances, it is important that the voltage at dielectric breakdown(BDV=break down voltage) is high, and therefore, dielectric ceramicswith high BDVs have been required.

Core-shell materials are suitable for applications in which relativelylarge voltages are applied. Dielectric ceramics including bothcrystalline grains which have a core-shell structure and crystallinegrains which have a homogeneous structure have been used when inaddition to a high BDV, good electronic characteristics (dielectricconstant, the temperature characteristics of capacitance, the lifetimecharacteristics in a high temperature load test, etc.) are desired.

For example, Japanese Patent No. 3376963 (Patent Document 1) andJapanese Patent No. 3793697 (Patent Document 2) disclose dielectricceramics including both crystalline grains which have a core-shellstructure and crystalline grains which have a homogeneous structure, andlaminated ceramic capacitors using the dielectric ceramics.

The dielectric ceramic used in the laminated ceramic capacitor disclosedin Patent Document 1 has a mixture of grains which have a core-shellstructure and grains which have a homogeneous structure, and when anycross section of the ceramic sintered body is observed, the grains whichhave the core-shell structure and the grains which have the homogeneousstructure are present at an area ratio in the range of 2:8 to 4:6.According to this dielectric ceramic, it seems that the relativedielectric constant is increased to 4500 or more, and a laminatedceramic capacitor using this dielectric ceramic satisfies Dcharacteristics as temperature characteristics of capacitance inaccordance with the JIS standard (Japanese industrial standard).

On the other hand, the dielectric ceramic of the laminated ceramiccapacitor disclosed in Patent Document 2 has a shell thickness whichchanges as the grain which has the core-shell structure becomes closerto a conductive layer, and grains which have no core-shell structure,that is, which have the homogeneous structure, are present at theinterface between the layer and the conductor, and the ratio between thegrains which have the core-shell structure and the number of the grainswhich have no core-shell structure is 7:3 or more and 9:1 or less. Itseems that the employment of such a configuration improves the withstandvoltage characteristics.

However, the dielectric ceramics disclosed in each of Patent Documents 1and 2 described above still have problems to be solved.

More specifically, the dielectric ceramic described in Patent Document 1can be further improved with respect to the temperature characteristicsof capacitance, as it fails to satisfy the X5R characteristics inaccordance with the EIA standard. Furthermore, the increase in the ratioof the crystalline grains which have the homogeneous structure easilyresults in grain growth, in particular, when the layer is made furtherthinner, and the lifetime characteristics may be decreased.

While it seems that the dielectric ceramic described in Patent Document2 improves the withstand voltage characteristics, that withstand voltagecharacteristics is still unsatisfactory when the layer is made furtherthinner. Furthermore, the desired lifetime characteristics is alsodifficult to achieve.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to provide a dielectricceramic, and a laminated ceramic capacitor configured with the use ofthe dielectric ceramic, which can solve the problems described above.

This invention has a feature in that a dielectric ceramic includescore-shell crystalline grains which have a core-shell structure andhomogeneous crystalline grains which have a homogeneous structure, andwherein the core-shell crystalline grains and the homogeneouscrystalline grains are present at an area ratio in the range of 91:9 to99:1.

When the mean grain size for the core-shell crystalline grains isrepresented by R1, and the mean grain size for the homogeneouscrystalline grains is represented by R2 in the dielectric ceramicaccording to the present invention, it is preferable that the ratio ofR2/R1 is 0.8 or more and 3 or less.

The dielectric ceramic according to the present invention typically hasa composition containing ABO₃ being Ba, and may further be at least oneof Ca and Sr; B is Ti, and may further be at least one of Zr and Hf) asa main component, and R (R is at least one selected from La, Ce, Pr, Nd,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) or Mg as an accessorycomponent.

The present invention is further directed to a laminated ceramiccapacitor including: a capacitor main body composed of a plurality oflaminated dielectric ceramic layers, and a plurality of internalelectrodes at interfaces between the dielectric ceramic layers; and aplurality of external electrodes formed in positions different from eachother on the outer surface of the capacitor main body, and electricallyconnected to specific ones of the internal electrodes.

The laminated ceramic capacitor according to the invention has a featurein that a dielectric ceramic layer is composed of the dielectric ceramicaccording to the invention as described above.

According to the dielectric ceramic according to the invention, thecore-shell crystalline grains and the homogeneous crystalline grains arepresent at an area ratio in the range of 91:9 to 99:1. Therefore, thelifetime characteristics and BDV can be more improved. Furthermore, therelatively small amount of homogeneous crystalline grains can thus makethe temperature characteristics more favorable.

In the dielectric ceramic according to the invention, when the ratioR2/R1 of the mean grain size R2 for the homogeneous crystalline grainsto the mean grain size R1 for the core-shell crystalline grains is 0.8or more and 3 or less, the lifetime characteristics and BDV can befurther improved.

Accordingly, the laminated ceramic capacitor configured with the use ofthe dielectric ceramic according to the invention can make thedielectric ceramic layers thinner while maintaining high reliability,thereby allowing the laminated ceramic capacitor to be reduced in sizeand increased in capacitance, as well as allowing the lifetimecharacteristics and temperature characteristics to be made better.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating a laminatedceramic capacitor 1 configured with the use of a dielectric ceramicaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

First, a laminated ceramic capacitor 1 to which a dielectric ceramicaccording to the invention is applied will be described with referenceto FIG. 1.

The laminated ceramic capacitor 1 includes a capacitor main body 5composed of a plurality of laminated dielectric ceramic layers 2 and aplurality of internal electrodes 3 and 4 along different interfacesbetween the dielectric ceramic layers 2. The internal electrodes 3 and 4contain, for example, a base metal such as Ni, as its main component.

First and second external electrodes 6 and 7 are formed in positionsdifferent from each other on the outer surface of the capacitor mainbody 5. The external electrodes 6 and 7 can contain Ag, Cu, or Ag—Pd astheir main components. In the laminated ceramic capacitor 1 shown inFIG. 1, the first and second external electrodes 6 and 7 are formed oneach of end faces of the capacitor main body 5 opposed to each other.The internal electrodes 3 and 4 include a plurality of first internalelectrodes 3 electrically connected to the first external electrode 6and a plurality of second internal electrodes 4 electrically connectedto the second external electrode 7, and these first and second internalelectrodes 3 and 4 are interlaminated in the laminate direction.

In this laminated ceramic capacitor 1, the dielectric ceramicconstituting the dielectric ceramic layers 2 has a feature in that thedielectric ceramic includes core-shell crystalline grains which have acore-shell structure and homogeneous crystalline grains which have ahomogeneous structure, and the core-shell crystalline grains and thehomogeneous crystalline grains are present at an area ratio in the rangeof 91:9 to 99:1.

It is to be noted that the “core-shell structure” mentioned above refersto a state in which an accessory component is present as a solidsolution in a surface layer portion of the crystalline grain, whereas aregion in which no accessory component is present as a solid solution ispresent in the center portion thereof. Furthermore, the “homogeneousstructure” refers to a state in which the accessory component is presenthomogeneously as a solid solution in the entire crystalline grain.

With the dielectric ceramic described above, the lifetimecharacteristics and BDV of the laminated ceramic capacitor 1 can be moreimproved, as is clear from experimental examples described below.Furthermore, the relatively small amount of homogeneous crystallinegrains can thus make the temperature characteristics more favorable.

In order to further improve the lifetime characteristics and BDVmentioned above, when the mean particle size for the core-shellcrystalline grains is represented by R1 and the mean grain size for thehomogeneous crystalline grains is represented by R2, it is preferablethat the ratio of R2/R1 be 0.8 or more and 3 or less.

The dielectric ceramic described above typically has a compositioncontaining ABO₃ (A being Ba, and may further contain at least one of Caand Sr; B being Ti, and may further contain at least one of Zr and Hf)as the main component, and R (R is at least one selected from La, Ce,Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) or Mg as theaccessory component.

Experimental examples carried out based on the invention will bedescribed below.

Experimental Example 1 (A) Manufacture of Dielectric Ceramic RawMaterial

For manufacturing a dielectric ceramic raw material, respective powdersof BaTiO₃, Dy₂O₃, MnCO₃, MgCO₃, SiO₂, BaCO₃, and(Ba_(0.96)Dy_(0.04))(Ti_(0.97)Mg_(0.03))O₃ were prepared. It is to benoted that the BaTiO₃ powder was prepared with the mean grain size of195 nm through SEM observation and the(Ba_(0.94)Dy_(0.06))(Ti_(0.97)Mg_(0.03))O₃ powder was prepared with themean grain size of 333 nm through SEM observation.

Next, Dy₂O₃, MnCO₃, MgCO₃ and SiO₂ were weighed to reach 2 moles, 1mole, 2 moles, and 1 mole, respectively, with respect to 100 moles ofBaTiO₃. Furthermore, (Ba_(0.94)Dy_(0.06))(Ti_(0.97)Mg_(0.03))O₃ wasweighed have the ratio shown in the column“(Ba_(0.94)Dy_(0.06))(Ti_(0.97)Mg_(0.03))O₃ Powder/Mix Ratio” of Table1, with respect to the BaTiO₃. Furthermore, BaCO₃ was weighed to reachBa/Ti=1.015.

Next, after mixing the respective powders of BaTiO₃, Dy₂O₃, MnCO₃,MgCO₃, SiO₂, BaCO₃, and (Ba_(0.96)Dy_(0.04))(Ti_(0.97)Mg_(0.03))O₃weighed as described above, pure water (water injection ratio: 1.2) wasadded, and grinding was then carried out in a forced circulation typewet grinding mill (with the use of PSZ media 0.3 mm in diameter).

Next, the ground powders was put into an oven set at a temperature of140° C. and dried for 8 hours to obtain a dielectric ceramic raw powder.

(B) Manufacture of Laminated Ceramic Capacitor

Next, a polyvinyl butyral binder and an organic solvent such as ethanolwere added to the dielectric ceramic raw material powder for wet mixingin a ball mill, thereby preparing ceramic slurry.

The ceramic slurry was subjected to sheet forming by the doctor blademethod to obtain rectangular ceramic green sheets.

Next, a conductive paste containing Ni was screen-printed on ceramicgreen sheets described above to form a conductive paste film to serve asinternal electrodes.

Next, 100 ceramic green sheets with the conductive paste films formedthereon were laminated so that the opposed sides from which theconductive paste films are drawn are alternated, thereby obtaining agreen laminated body to serve as a capacitor main body.

Next, the green laminated body was heated to a temperature of 250° C. ina N₂ atmosphere, and subjected to processing for removing the binder,and then a calcination step was carried out under the conditions ofmaintaining a top temperature of 1230° C. and a partial oxygen pressureof 10^(−9.5) Pa for 120 minutes in a reducing atmosphere composed of aH₂-N₂-H₂O gas to sinter the raw laminated body, thereby obtaining acapacitor main body.

Next, an Ag—Pd paste containing glass frit was applied to the opposededge surfaces of the obtained laminated body, and baked at a temperatureof 800° C. in a N₂ atmosphere to form external electrodes electricallyconnected to the internal electrodes, thereby obtaining laminatedceramic capacitors as samples.

The laminated ceramic capacitor thus obtained had outer dimensions of awidth 1.6 mm, a length 3.2 mm, and a thickness 0.8 mm, and the thicknessof the dielectric ceramic layer interposed the internal electrodes was2.5 μm.

(C) Analysis of Ceramic Structure

For the laminated ceramic capacitors according to the respective samplesobtained, mapping analysis for Dy and Mg was carried out by TEM-EDX atpolished ceramic cross section. In this analysis, the region to beanalyzed was prepared to include at least 100 crystalline grains, thebeam spot diameter was made 1 nm, and the number of acquisitions was setto be 30 at 0.5 msec/point.

In the results of the analysis described above, the crystalline grainsin which Dy or Mg is homogeneously present are referred to as“homogeneous grains”, whereas the crystalline grains in which a regionin which none of Dy and Mg is present is located in the center portionare referred to as “core-shell crystalline grains”. The areas of therespective grains were calculated by image analysis to obtain the arearatio.

This area ratio is shown in the column “Sintered Body AreaRatio/Core-Shell:Homogeneous” of Table 1.

Furthermore, the mean grain size R1 for the “core-shell crystallinegrains” and the mean grain size R2 for the “homogeneous crystallinegrains” were obtained by similar image processing. These mean grainsizes “R1” and “R2” and “R2/R1” are shown in Table 1.

(D) Evaluation of Electrical Characteristics

For the laminated ceramic capacitors according to the respective samplesobtained, the dielectric constant, temperature characteristics,accelerated life, and BDV were evaluated as shown in Table 1.

The dielectric constant was calculated from the capacitance measured at0.5 Vrms and 1 kHz.

For the temperature characteristics, the rate of change in capacitancein the range of −55° C. to 125° C. was obtained with the capacitance at25° C. The maximum rate of change is shown in Table 1.

For the accelerated life, a high temperature load test of applying avoltage of 100 V (electric field strength: 40 kV/mm) at a temperature of150° C. was carried out for the 20 samples, and breakdown defined aswhen the insulation resistance value was less than 10 kΩ. The 50%frequency down time obtained from the Weibull distribution is shown inTable 1.

For the BDV, the voltage was gradually increased, and the voltage at thepoint of dielectric breakdown (10 kΩ or less) was obtained.

(E) Results

TABLE 1 (Ba₀ _(.) ₉₄Dy_(0.06)) Sintered Body (Ti₀ _(.) ₉₇Mg_(0.03))O₃Area Ratio Temperature Accelerated Sample Powder Mix Ratio Core-Shell:Dielectric Characteristics Life BDV Number (weight ratio) Homogeneous R1(nm) R1 (nm) R2/R1 Constant (%) (hours) (V) 1 100:0  100:0  189 — 0 1821−13.7 349 480 2 99:1 99:1 210 320 1.5 1830 −13.7 457 564 3 97:3 97:3 200351 1.8 1810 −14.0 553 581 4 95:5 95:5 198 284 1.4 1795 −14.3 652 584 593:7 93:7 215 332 1.5 1801 −14.7 639 600 6 91:9 91:9 209 301 1.4 1790−14.9 534 553 7  90:10  90:10 207 307 1.5 1713 −15.1 425 478

As seen from Table 1, samples 2 to 6 with the “Sintered Body AreaRatio/Core-Shell:Homogeneous” in the range of 91:9 to 99:1 achievedhigher-quality results for both of the accelerated life and BDV, ascompared with the samples 1 and 7 which deviated from this range.

Furthermore, samples 2 to 6 described above also satisfied X7Rcharacteristics for the “temperature characteristics”.

Experimental Example 2

Laminated ceramic capacitors as samples were manufactured under the sameconditions as in Experimental Example 1, and evaluated in the same wayas in Experimental Example 1, except that:

(1) the mean particle size of prepared BaTiO₃ powder through SEMobservation and the mean particle size of prepared(Ba_(0.96)Dy_(0.04))(Ti_(0.97)Mg_(0.03))O₃ powder through SEMobservation are shown respectively in the columns “BaTiO₃ Powder/MeanParticle Size” and “(Ba_(0.96)Dy_(0.04))(Ti_(0.97)Mg_(0.03))O₃Powder/Mean Particle Size”; and

(2) the mix ratio of (Ba_(0.94)Dy_(0.06))(Ti_(0.97)Mg_(0.03))O₃ toBaTiO₃ (corresponding to “(Ba_(0.94)Dy_(0.06))(Ti_(0.97)Mg_(0.03))O₃Powder/Mix Ratio” of Table 1) was fixed at 95:5. The evaluation resultsare shown in Table 2.

TABLE 2 (Ba₀ _(.) ₉₄Dy_(0.06)) Sintered Body BaTiO₃ (Ti₀ _(.)₉₇Mg_(0.03))O₃ Area Ratio Temperature Accelerated Sample Powder MeanPowder Mean Core-Shell: Dielectric Characteristics Life BDV NumberParticle Size (nm) Particle Size (nm) Homogeneous R1 (nm) R1 (nm) R2/R1Constant (%) (hours) (V) 11 214 666 95:5 214 684 3.2 1832 −13.7 452 50412 206 588 95:5 206 608 3.0 1814 −13.6 531 551 13 200 454 95:5 200 4462.2 1840 −14.0 649 571 14 215 238 95:5 215 220 1.0 1820 −14.3 673 604 15209 166 95:5 209 167 0.8 1850 −14.6 574 563 16 207 125 95:5 207 122 0.61902 −148 564 511

As can be seen when the samples 12 to 15 are compared with the samples11 and 16 in Table 2, it has been confirmed that the accelerated lifeand BDV can be kept at high levels when the ratio R2/R1 of the meangrain size R2 of the crystalline grains with the homogeneous structureto the mean grain size R1 of the crystalline grains with the core-shellstructure is made 0.8 or more and 3 or less.

1. A dielectric ceramic comprising crystalline grains which have acore-shell structure and crystalline grains which have a homogeneousstructure, wherein the core-shell crystalline grains and the homogeneouscrystalline grains are present at an area ratio in the range of 91:9 to99:1.
 2. The dielectric ceramic according to claim 1, wherein the ratioR2/R1 in which R2 is the mean grain size for the homogeneous crystallinegrains and R1 is the mean grain size for the core-shell crystallinegrains, is 0.8 or more and 3 or less.
 3. The dielectric ceramicaccording to claim 2, wherein the core-shell crystalline grains tohomogeneous crystalline grains area ratio is the range of 93:7 to 97:3.4. The dielectric ceramic according to claim 3, having a compositioncomprising ABO₃ in which A comprises Ba, and optionally contains atleast one of Ca and Sr, and B comprises Ti, and optionally contains atleast one of Zr and Hf, as a main component, and R or Mg as an accessorycomponent wherein R is at least one member selected from La, Ce, Pr, Nd,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
 5. The dielectric ceramicaccording to claim 4, wherein A is Ba and B is Ti, and the accessorycomponent is Dy or Mg.
 6. The dielectric ceramic according to claim 1,wherein the core-shell crystalline grains to homogeneous crystallinegrains area ratio is the range of 93:7 to 97:3.
 7. The dielectricceramic according to claim 6, having a composition comprising ABO₃ inwhich A comprises Ba, and optionally contains at least one of Ca and Sr,and B comprises Ti, and optionally contains at least one of Zr and Hf,as a main component, and R or Mg as an accessory component wherein R isat least one member selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, and Lu.
 8. The dielectric ceramic according to claim 7,wherein A is Ba and B is Ti, and the accessory component is Dy or Mg. 9.The dielectric ceramic according to claim 1, having a compositioncomprising ABO₃ in which A comprises Ba, and optionally contains atleast one of Ca and Sr, and B comprises Ti, and optionally contains atleast one of Zr and Hf, as a main component, and R or Mg as an accessorycomponent wherein R is at least one member selected from La, Ce, Pr, Nd,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
 10. The dielectric ceramicaccording to claim 9, wherein A is Ba and B is Ti, and the accessorycomponent is Dy or Mg.
 11. The dielectric ceramic according to claim 1,wherein A is Ba and B is Ti, and the accessory component is Dy or Mg.12. A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim 11.13. A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim
 9. 14.A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim
 6. 15.A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim
 5. 16.A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim
 4. 17.A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim
 3. 18.A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim
 2. 19.A laminated ceramic capacitor comprising: a capacitor main bodycomprising a plurality of laminated dielectric ceramic layers, and aplurality of internal electrodes disposed at different interfacesbetween adjacent dielectric ceramic layers; and a pair of externalelectrodes disposed at positions different from each other on an outersurface of the capacitor main body, and electrically connected todifferent ones of the internal electrodes, wherein the dielectricceramic layers comprise the dielectric ceramic according to claim 1.